This invention relates to an electrocatalytic structure and a method of preparing such structure. The electrocatalytic structure can be employed in various chemical reactions such as for example to catalyze the electrochemical reactions taking place in fuel cells employing gaseous fuels. In another aspect, the invention concerns fuel cells (especially fuel tells which contain a solid polymer electrolyte) which include the novel electrocatalytic structure as an element thereof.
Electrochemical reactions, such as those carried out in energy conversion devices such as fuel cells containing a solid polymer electrolyte membrane (proton exchange membrane) have been studied extensively since the early 1960's. These fuel cells contain an anode compartment and a cathode compartment separated by the solid polymer electrolyte. Gaseous fuels, such as H.sub.2 and methanol, used in these fuel cells require a catalyst to promote the half cell reactions to proceed at useful rates. The catalyst structure should possess several characteristics to be efficient and effective. There should be a low catalyst loading (low weight per active surface area ratio) to reduce the cost of producing electricity. There should be efficient proton and gas access to the catalyst. There should be electronic continuity and low internal electrical resistance. Low susceptibility to carbon monoxide poisoning is important. The catalyst structure must have physical and chemical compatibility with the other components of the cell, e.g., anode and cathode electrodes. The catalyst material should preferably maintain an effective high surface area, e.g., not coalesce, at a temperature up to at least about 140.degree. C. Moreover, the presence of water should not block the gas flow through the structure.
The present invention increases the utility of the catalysts in fuel cells and other electrochemical reactions by improving the effectiveness and efficiency of each of these criteria.
The preparation of composite structures containing a matrix having dispersed therein nanometer sized particles of a material different from the matrix have been prepared and used for different purposes. For example, in U.S. Pat. No. 5,158,933, a phase-separated composite film of two different inorganic materials is disclosed. In that patent, a continuous layer of a first element is prepared by sputter deposition at a first sputtering pressure which is sufficiently low to form a continuous layer (matrix). A layer of discrete particles of a second element is then sputtered onto the first continuous layer under conditions permitting the formation of particles instead of a continuous layer. The layer of discrete particles is then coated with a second continuous layer (matrix) of the first element by adjusting the conditions of sputtering to those used in the first step. A plurality of alternating layers of particles and continuous phases can be built up by repeating the process steps. The process is used to prepare composite structures of, for example, discrete particles of molybdenum dispersed in a continuous aluminum matrix. The composites are used for various electronic, magnetic and glass ceramic applications such as high temperature coatings and the like. The process of this patent is particularly useful for making structures containing two different elements which are normally miscible under conditions of concurrent sputtering.
The Journal of Applied Phys., Vol. 44, No. 6, June 1973, contains an article entitled "Optical Properties of Electrically Insulating Granular Metal Films" by H. R. Zeller and D. Kuse at pages 2763 and 2764, which describes a structure composed of metal particles and an inorganic dielectric. The structure is prepared by successive evaporation of metal, e.g., tin, and an insulator, e.g., MgF.sub.2 to form a structure containing islands (particles of tin) and a layer of an insulator, e.g. MgF.sub.2. The small metal particles are insulated from each other.
In another field, an optical information storage media is prepared by vacuum depositing (e.g., sputtering) a discontinuous film of nucleated metallic islands (particles)onto a substrate. The islands have a thickness of less than about 100 Angstroms (10 nanometers) and a diameter measured parallel to the surface of the substrate of less than about 1000 Angstroms (100 nanometers). The particles are then coated with a transparent dielectric material such as a polymer using, for example, glow discharge techniques.
Another technique, used to prepare colored polymeric coatings, is taught by H. A. Beale, "Plasma Polymerization Produces Colored Polymeric Coatings", Industrial Research and Development, July 1981, Page 135-139. Plasma polymerization of a suitable monomer is carried out with evaporation of a metal to provide a metal particle loaded film.
Many electrochemical reactions and systems are disclosed which utilize a catalyst in the system. For example, see U.S. Pat. Nos. 4,039,409; 5,084,144; 4,876,115; 5,302,269 and 5,151,515. Several publications describe work with catalyst systems for use in solid polymer electrolyte fuel cells. For example, see page 137-152 of Energy, Special Issue, "Assessment of Research Needs For Advanced Fuel Cells", Pergamon Press, Editor, S. S. Penner, ISBN 0-08-033990-5 (1986); J. Electroanal. Chem. 251 (1988), page 275-295, "Localization of Platinum In Low Catalyst Loading Electrodes to Attain High Power Densities in SPE Fuel Cells", Edson A. Tieianelli, Charles R. Deronium and Supramaniama Srinivasan, Los Alamos National Laboratory (1988); Journal of Applied Electrochemistry, vol. 22, p. 127, (1992), "Thin Film Catalyst Layers for polymer Electrolyte Fuel Cell Electrodes", M. S. Wilson and S. Gottesfild. The electrocatalytic structures of the invention have superior properties over those taught in this last publication.